Myosins are the molecular motors essential for the viability of eukaryotic cells. In particular, non-muscular class V myosins are required for the inheritance, positioning, and transport of organelles in yeast, animals, and humans, whereas plants possess closely related class XI myosins. We have demonstrated that plant myosins empower trafficking of the Golgi stacks, peroxisomes and mitochondria. Furthermore, we have discovered novel myosin functions in Factin organization, overall cell expansion, polarized cell elongation, and growth of an entire organism. Our proposal is developed to address the mechanisms of these fundamental myosindependent processes relevant to broad areas of eukaryotic cell biology and medicine. We have established plants as an advanced model for studying myosin functions in the higher eukaryotes and generated tools and technologies required for the successful completion of the project. Our preliminary research highlighted striking parallels in myosin functions between plants and vertebrates. Therefore, we are uniquely positioned to complete the research under two Specific Aims that do not depend on each other, but rather have a synergistic nature. Aim 1. To determine roles of myosins in cell growth and polarized elongation. This will be done via investigation of the cell structure and dynamics in multiple knockout mutants. The correlation analysis of the myosin activities in F-actin organization, organelle trafficking, overall cell expansion, elongation of polarized cells, as well as plant growth and development will be used to advance a testable mechanistic model. Aim 2. To investigate mechanisms that control cargo recognition by the myosins. The organelle-specific myosin receptors and associated regulatory proteins will be identified using imaging-based genetic screens and bioinformatics. The functions of the receptor complex components will be determined using phenotypic analysis of null mutants. Together with Aim 1, this analysis will substantially contribute to the development of a general mechanistic model of the myosin-driven cell interior dynamics. The proposed research will take advantage of the extensive genomic, bioinformatics, and cell biology resources available for a model plant Arabidopsis. Our studies into the myosin function in organelle transport, F-actin organization, polarized cell elongation, and determination of the organism size and reproductive success have opened a new research avenue. The conceptual and technical advances developed in Arabidopsis model will inform and facilitate the progress in the vertebrate models. Due to increasing impact on human health and disease, from memory formation and neurological disorders to fatal defects in cell differentiation to viral infection, the health-related implications of the myosin research in all eukaryotic models including Arabidopsis are certain to grow.